1. Field of the Invention
This invention pertains to the general field of optical metrology. In particular, it pertains to a fixture for flattening large samples being measured with an optical instrument such as an interferometer.
2. Description of the Prior Art
When measuring a relatively large sample such as a flexible circuit-board panel with an optical instrument, it is very desirable to hold the panel as flat as possible. Such samples are measured at a plurality of locations distributed over the surface of the panel and each measurement is preceded by a focusing procedure whereby the objective of the instrument is placed at a best-focus position prior to capturing the optical information constituting the measurement. Therefore, a perfectly flat sample greatly reduces the time required to focus the objective on the sample surface.
Various mechanisms have been used in the art in order to flatten large, flexible, panel-like samples. The typical sample stage consists of a platform over which the sample rests during the measurement, normally clamped at the edges to keep it in place under conditions that minimize contact of its bottom surface with the support structure. When the sample is bowed downward producing a lower convex surface, it can be flattened by a simple mechanical support that keeps the center of the sample roughly coplanar with the outer edges. Such a mechanism could be, for example, a thin and rigid cross-like structure over which the sample bears when clamped to the stage. When the sample is bowed upward, however, its center cannot be brought down from the top without significant interference from the actuating mechanism, which is cumbersome and undesirable.
Vacuum devices are known in the art for flattening large samples by drawing them to a flat support platter so as to conform to its surface. Channels in the platter provide the means by which the vacuum is exerted on the sample after it contacts the platter. However, these devices are unsuitable for circuit boards that require a minimum amount of contact with the flattening fixture and only with particular materials, such as Teflon® (PTFE—polytetrafluoroethylene) and other plastic or “non-marring” materials, that have been found not to produce damage on contact. This invention describes a suction device that flattens upwardly bowed samples with minimum contact and no damage to the bottom surface of the sample.